[tests] allow BOOST_CHECK_EQUAL for ScriptErrors

Summary:
as per title -- BOOST_*_EQUAL needs operator<< to be defined for
the type so it can print something like "A != B".

Test Plan: ninja check

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, deadalnix, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4798